The present invention relates to the field of angioplasty and in particular, to new and improved catheters for performing balloon angioplasty procedures on blood vessels.
Angioplasty has gained wide acceptance in recent years as an efficient and effective method for treating vascular diseases. In particular, angioplasty is widely used for opening a stenosis in the coronary arteries, and is also used for treatment of a stenosis in other parts of the vascular system.
The most widely used method of angioplasty makes use of a dilatation catheter which has an inflatable balloon at a distal end. Using an x-ray fluoroscope, a physician guides the catheter through the vascular system until the balloon is positioned across the stenosis. The balloon is subsequently inflated, exerting pressure radially and outwardly against the stenosis, causing the artery wall to stretch and re-establishing an acceptable blood flow through the artery.
In order to treat a stenosis having a very small opening, there has been continuing efforts to reduce the profile of dilatation catheters so that the catheter not only can reach but also can cross such a very tight stenosis. An important factor in determining the profile of a dilatation catheter is the wall thickness of the balloon material. Another important factor concerning profile is the diameter and flexibility of the proximal and distal ends of the balloon. A balloon having a reduced diameter and a more flexible distal end permits an easier "start" into the stenosis and thus provides a greater likelihood that the balloon can be properly positioned across the stenosis.
A typical method of manufacturing a balloon for use in a dilatation catheter generally begins by extruding a tubing material from an extruder using a particular draw-down ratio and rate of extrusion. The size of the extruder, draw-down ratio, and rate of extrusion determine the inner and outer dimensions of the tubing material. The dimensions are adjusted according to a desired balloon size (e.g., 2.0, 2.5, 3.0, 3.5 mm) and desired balloon burst strength (e.g., 12 or 14 atm).
The extruded tubing, upon cooling, is irradiated (e.g., exposed to electron beam radiation) which changes the mechanical properties of the tubing to increase its strength and control compliance characteristics of the balloon. The tubing is cut into segments and placed into a blow-mold cavity wherein an intermediate segment of the tubing is heated and pressurized to expand radially outwardly, thereby becoming a distensible main body portion of the balloon. The distal and proximal "waists" of the tubing/balloon are also heated and blown out in the mold, as needed, in order to get the inner diameters of the waists to suitable dimensions for bonding those ends onto the other catheter components. Thus, the balloon has narrow distal and proximal segments relative to an enlarged main body portion.
Once the balloon is formed, a proximal end thereof is attached to a distal end of a catheter tube. Typically, a proper bond is achieved by overlapping a distal end segment of the catheter tube with a larger diameter proximal end segment of the proximal waist of the balloon, and providing a suitable adhesive bond therebetween.
Current balloon manufacturing methods require that each balloon size and strength combination utilize a tubing material with specific inner and outer dimensions. This requirement allows proper formation of the distensible portion of the balloon. One drawback to this situation is that the proximal and distal segments of the tubing element forming the balloon are restricted to the dimensions of the original tubing material. Also, during the heating and pressurizing process used to form the distensible portion of the balloon, the internal heat and pressure consequently increase the dimensions of the proximal and distal segments of the tubing element in order to get the desired inner diameter. Thus, the proximal and distal segments typically end up having diameters which are larger than desired, and potentially larger than what the folded balloon profile can be when fully folded down. This problem becomes more pronounced as catheter tube diameters are reduced in an effort to minimize catheter profiles and when larger distensible balloon portions are needed (because the dimensions of original tubing must increase as the distensible portion of the balloon is increased). In other words, the problems of large diameter waist segments becomes more pronounced as the other catheter elements become smaller.
Another disadvantage from balloons formed by current methods stems from the thickness of the proximal and distal ends. When the intermediate segment is heated and pressurized, the balloon walls forming the distensible portion of the balloon become thinner. This thinning is taken into account when determining the variables (rate of the extrusion, extruder size, and draw-down ratio) of the extrusion process. Thus, the balloon material is extruded at a thickness which is greater than the finished balloon. Because the proximal and distal ends will expand only slightly during balloon formation (with a slight decrease in wall thickness), the thicker material in the proximal and distal ends reduce the flexibility of the balloon and contributes to a larger balloon profile.
In an effort to reduce balloon profile and to protect the balloon, a protective sleeve, preferably formed of plastic having an inner surface coated with a lubricious coating, is placed over the balloon. The sleeve is threaded onto the balloon from its distal end, causing the walls of the balloon to fold and wrap tightly around the guide wire (in a fixed-wire catheter) or an inner core which contains the guide wire (in an over-the-wire catheter). The sleeve is removed just prior to inserting the catheter into a patient. The compression caused by the sleeve reduces the balloon's profile allowing the catheter to cross tighter and tighter lesions. It is desirable to have the distal balloon waist considerably smaller than the diameter of a fully "compressed" or "wrapped" balloon. Having the waist smaller means that the "crossing profile" of the balloon is determined by the diameter of the folded balloon diameter, rather than the diameter of the distal waist outer diameter. A thicker and less flexible distal waist thus prohibits smaller diameter sleeves from being placed onto the balloon, and causes an unnecessary increase in balloon profile because the balloon cannot be fully compressed.
Another disadvantage concerns crossing the stenosis once the sleeve is removed. Thinner and more flexible proximal and distal waists on the balloon allow an easier "start" into the stenosis, providing a better chance that the balloon can be effectively advanced across the lesion. On the other hand, a thicker distal waist causes the balloon to bunch as it enters the stenosis, further reducing the ability of the balloon to be properly positioned.
The disadvantages described above have presented substantial difficulties for dilatation balloon catheter manufacturers working to reduce the profiles of catheter balloons. The disadvantages are especially prevalent in fixed-wire catheters which are typically designed for tight stenosis procedures. Thus, there is a need to manufacture a balloon having a distensible portion with the proper size and strength characteristics for dilation procedures while providing flexible small diameter proximal and distal ends to minimize the disadvantages described above. Heretofore, however, manufacturing techniques to accomplish this goal had not been developed.